1. Field of the Invention
The invention generally relates to a microlithographic apparatus, for example a projection exposure apparatus or a mask inspection apparatus. The invention relates in particular to such an apparatus comprising a wavefront correction device in which heating light distinct from projection light is directed towards a rim surface of a mirror substrate.
2. Description of Related Art
Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A microlithographic projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, the more devices can be produced on a single wafer, the higher is the throughput of the apparatus.
The size of the structures which can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelength currently used is 193 nm, which is in the vacuum ultraviolet (VUV) spectral range. Also apparatus using EUV light having a wavelength of about 13.5 nm are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as about 6.8 nm. Since there are no optical materials available which are sufficiently transparent for EUV projection light, such apparatus are of the catoptric type, i.e. they contain only mirrors, but no lenses or other refractive optical elements.
The correction of image errors (i.e. aberrations) is becoming increasingly important for projection objectives with very high resolution. Different types of image errors usually require different correction measures.
The correction of rotationally symmetric image errors is comparatively straightforward. An image error is referred to as being rotationally symmetric if the wavefront error in the exit pupil of the projection objective is rotationally symmetric. The term wavefront error refers to the deviation of an optical wavefront from the ideal aberration-free wavefront. Rotationally symmetric image errors can be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of those image errors which are not rotationally symmetric is more difficult. Such image errors occur, for example, because lenses or mirrors heat up rotationally asymmetrically. One image error of this type is astigmatism.
A major cause for rotationally asymmetric image errors is a rotationally asymmetric heating of the optical elements.
For example, in projection exposure apparatus of the scanner type the field that is illuminated on a mask is usually slit-shaped. This slit-shaped illuminated field causes a nonuniform heating of those optical elements that are arranged in the vicinity of field planes. This heating, in turn, results in deformations of the optical elements and, in the case of lenses and other elements of the refractive type, in changes of their refractive index. If the materials of refractive optical elements or of mirror substrates are repeatedly exposed to the high energetic projection light, also permanent material changes are observed. For example, a compaction of the materials exposed to the projection light sometimes occurs. In the case of refractive optical elements this may result in a local and permanent change of the refractive index, and in the case of mirror substrates in a local and permanent change of the surface shape of the substrate. Apart from that, the very complex and expensive reflective multi-layer coatings of EUV mirrors may be damaged by high local light intensities so that the reflectance is locally altered. The same also applies to the anti-reflection coatings that are usually applied on the optical surfaces of lenses and other refractive optical elements.
Another major cause for rotationally asymmetric heating are certain asymmetric illumination settings in which the pupil plane of the illumination system is illuminated in a rotationally asymmetric manner. Important examples for such settings are dipole settings in which only two poles are illuminated in the pupil plane. With such a dipole setting, also the pupil planes in the projection objective contain two strongly illuminated regions. Consequently, lenses or mirrors arranged in or in the vicinity of such a pupil plane are exposed to a rotationally asymmetric intensity distribution giving rise to rotationally asymmetric image errors. Also quadrupole settings often produce rotationally asymmetric image errors, although to a lesser extent than dipole settings.
Generally, heat induced deformations, index changes and coating damages alter the optical properties of the optical elements and thus cause image errors. Heat induced image errors sometimes have a twofold symmetry. However, image errors with other symmetries, for example threefold or fivefold, are also frequently observed in projection objectives.
In order to correct rotationally asymmetric image errors, U.S. Pat. No. 6,338,823 B1 proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. The deformation of the lens is determined such that heat induced image errors are at least partially corrected. However, if a lens is deformed, this involves necessarily the deformation of both optical surfaces of the lens. Usually the effects caused by each lens surface compensate each other to some extent so that only significant deformations produce the desired correction of the image error.
For that reason it has been proposed to use deformable mirrors to correct image errors. In EUV apparatus, in which refractive optical elements cannot be used, any deformable optical element has to be a mirror, anyway. Deformable mirrors for microlithographic apparatus are disclosed in U.S. Pat. No. 6,897,940 and U.S. Pat. No. 5,986,795, for example.
US 2010/0201958 A1 and US 2009/0257032 A1 disclose a correction device that comprises two transparent optical elements that are separated from each other by a liquid layer. In contrast to the device described in the aforementioned U.S. Pat. No. 7,830,611 B2, a wavefront correction is not obtained by deforming the optical elements, but by changing their refractive index locally. To this end one optical element may be provided with heating stripes that extend over the entire surface. The liquid ensures that the average temperatures of the optical elements are kept constant. It is also mentioned that the heating elements may be applied on or behind a reflective surface of a mirror. Although even higher order wavefront errors can be corrected very well, this device has a complex structure and is therefore expensive.
WO 2004/092843 A2 discloses a correction device for an EUV projection objective that directs correction light towards the reflective surface of a mirror. The correction light is controlled such that the temperature in the vicinity of the reflective surface comes close to the temperature where the coefficient of thermal expansion of the mirror substrate is zero.
EP 0 532 236 A1 discloses another correction device for an EUV projection objective. In one embodiment infrared radiation is directed on one of the mirrors of the objective. The infrared light is controlled such that the shape of the mirror does not substantially alter even under the impact of the high energy EUV projection light. In other embodiments heating or cooling devices are integrated into the mirror support for the same purpose.
U.S. Pat. No. 6,504,597 B2 proposes a correction device in which heating light is coupled into a lens or a mirror via its peripheral rim surface, i.e. circumferentially. Optical fibers may be used to direct the heating light produced by a single light source to the various locations distributed along the periphery of the optical element. It is also mentioned that this device may not only be used to homogenize the temperature distribution of the optical element, but also to correct wavefront errors caused in other optical elements. Although this device makes it possible to heat up also optical elements that are very densely stacked, it is only capable of producing comparatively coarse temperature distributions. More complex temperature distributions cannot be attained because only very few and strongly divergent heating light beams can be coupled into the optical element.